Secondary fastener forming machines take cylindrical workpieces, such as headed fastener blanks that have been formed in a primary fastener forming process, and perform additional operations on the workpieces such as tapering the ends. To perform additional operations such as pointing on the blanks, some secondary forming machines drive the blanks axially along a linear forming stroke into engagement with cutters mounted on a cutting head. It is highly desirable that secondary fastener forming machines of this type include means for adjusting the length of each forming stroke to accommodate fastener blanks and other workpieces of various lengths. It is also desirable that such machines include a mechanism that senses when a blank has reached the end of its forming stroke and causes the machine to reverse and disengage the formed workpiece from the forming tool.
Optical Boring Company, the assignee of the present invention, manufactures a secondary fastener forming machine (the OBC machine) that forms pointed ends onto headed fastener blanks. The OBC machine includes a reciprocator in the form of a 2-piston hydro-pneumatic cylinder that reciprocates a reciprocating portion of the assembly including a fastener gripper upward to a fully retracted "remote" position and downward through a cutting stroke to a fully extended "proximate" position. During the lower portion of the cutting stroke the lower end of a fastener blank held in the gripper axially engages a rotating cutting tool at a cutting tool rotational axis. The cutting tool includes carbide cutting blades that are positioned around the cutting tool axis to cut a tapered conical tip on the fastener blank. One piston within the cylinder is hydraulically dampened leaving no hard linkages between the forming machine and the fastener blank being worked. This lack of hard linkages allows the downward travel of the fastener blank and gripper to slow with increased resistance during the cutting stroke and to slow further or even stop completely in response to excessive resistance. After cutting, the cylinder lifts the gripper and extracts the fastener from the cutting tool, completing a cutting cycle. The OBC machine also includes a part transfer mechanism that inserts and ejects parts. The reciprocal motion of one of the pistons within the hydro-pneumatic cylinder drives the part transfer mechanism.
An adjustable screw axially engages a limit switch whenever a fastener has reached the end of its forming stroke. When the limit switch is thus activated it signals the reciprocator to reverse the reciprocating portion of the assembly and disengage the newly-tipped fastener from the cutters. The screw may be advanced or retracted to adjust the reversal point of the cylinder at the end of the cutting stroke.
While a reciprocator of this type provides many advantages it cannot consistently terminate the downward forming/cutting stroke at the same proximate position within acceptable tolerances. If cycle speed is increased for any reason, the reciprocating portion of the assembly and the fastener blank can overshoot the proximate position resulting in a deeper cut on the fastener. Conversely, if cycle speed decreases for any reason, the reciprocating portion may undershoot the desired reversal point resulting in a shallower cut on the fastener. Cycle speed will tend to decrease over time as the cutters begin to get dull from repeated cutting cycles. The cycle speed slows because the force required to advance each fastener blank into the cutters increases while the pneumatic pressure supply remains relatively constant. In addition, small variations in pneumatic pressure supply are common with most industrial air compressor systems and can result in cutting depth variations that are unacceptable in certain applications.
What is needed is a hydro-pneumatically driven fastener forming machine that, without hard linkages, moves fastener blanks through a forming stroke while positively and consistently maintaining the end-of-stroke stopping position, i.e., the "proximate" position of each workpiece relative to the forming tool. What is also needed is such a machine that can accommodate bolts of various lengths.